Effects of Planting Dates on Expression of Phytophthora Root Rot Resistance in Alfalfa. D. R. Porter, Graduate Research Assistant, Agronomy Department, Oklahoma State University, Stillwater 74078. J. L. Caddel, Associate Professor, Agronomy Department, and L. L. Singleton, Associate Professor, Department of Plant Pathology, Oklahoma State University, Stillwater 74078. Plant Dis. 70:655-657. Accepted for publication 3 February 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-655.

Four field studies were conducted to identify efficient screening techniques to maximize the expression of disease resistance and minimize the time required for one cycle of selection for resistance to Phytophthora root rot (PRR) caused by Phytophthora megasperma f. sp. medicaginis in alfalfa (Medicago sativa) under Oklahoma conditions. In 1981 and 1982, the effects of six and eight screening dates, respectively, from 2 November through 15 July, on severity of PRR symptoms of fall-planted alfalfa were evaluated. Disease severity indices and percent resistant plants of two PRR-resistant and two PRR-susceptible cultivars maintained under saturated soil conditions were used as indicators of screening effectiveness. Results indicated effective screening can be obtained by mid-May provided soil temperatures exceed 12 C before screening. In two separate studies, the effects of six spring planting dates (5 March through 13 May) were evaluated with an August screening in 1982 and 1983. However, none of the spring planting dates clearly allowed for differentiation between susceptible and resistant cultivars.